The present invention relates to disk drive data storage devices, and in particular, to the manufacture of glass or ceramic disks for use in disk drive data storage devices.
The latter half of the twentieth century has been witness to a phenomenon known as the information revolution. While the information revolution is a historical development broader in scope than any one event or machine, no single device has come to represent the information revolution more than the digital electronic computer. The development of computer systems has surely been a revolution. Each year, computer systems grow faster, store more data, and provide more applications to their users.
The extensive data storage needs of modem computer systems require large capacity mass data storage devices. While various data storage technologies are available, the rotating magnetic rigid disk drive has become by far the most ubiquitous. Such a disk drive data storage device is an extremely complex piece of machinery, containing precision mechanical parts, ultra-smooth disk surfaces, high-density magnetically encoded data, and sophisticated electronics for encoding/decoding data, and controlling drive operation. Each disk drive is therefore a miniature world unto itself, containing multiple systems and subsystem, each one of which is needed for proper drive operation. Despite this complexity, rotating magnetic disk drives have a proven record of capacity, performance and cost which make them the storage device of choice for a large variety of applications.
A disk drive typically contains one or more disks attached to a common rotating hub or spindle. Each disk is a thin, flat member having a central aperture for the spindle. Data is recorded on the flat surfaces of the disk, usually on both sides. A transducing head is positioned adjacent the surface of the spinning disk to read and write data. Increased density of data written on the disk surface requires that the transducer be positioned very close to the surface. Ideally, the disk surface is both very flat and very smooth. Any surface roughness or xe2x80x9cwavinessxe2x80x9d (deviation in the surface profile from an ideal plane) decrease the ability of the transducing heads to maintain an ideal distance from the recording media, and consequently decrease the density at which data can be stored on the disk.
The disk is manufactured of a non-magnetic base (substrate), which is coated with a magnetic coating for recording data on the recording surfaces, and which may contain additional layers as well, such as a protective outer coating. Historically, aluminum has been the material of choice for the substrate. As design specifications have become more demanding, it is increasingly difficult to meet them using aluminum, and in recent years there has been considerable interest in other materials, specifically glass. Glass or ceramic materials are potentially superior to aluminum in several respects, and offers the potential to meet higher design specifications of the future.
One of the major drawbacks to the use of glass or ceramic disk substrates is the cost of their manufacture. Glass is currently used in some commercial disk drive designs, although generally at a higher cost than conventional aluminum. In a typical glass disk manufacturing process, the glass base material is initially formed in thin glass sheets. Multiple glass disks are then cut from a sheet. The process of forming the glass sheets leaves some waviness in the glass, and so the disks are typically lapped to reduce the waviness. Lapping leaves a thin fracture layer near the surface of the glass disks, which is unsuitable for use in disk drives. The fracture layer is therefore removed by a rough polishing step. The disks are then subjected to a second, fine polishing step to remove scratches and minor imperfections left by the rough polishing step and to achieve a suitably smooth finish. The glass substrate thus formed is then coated with a magnetic recording layer, and may be coated with other layers such as a protective layer.
Each of these steps adds to the cost of the disk. In particular, the polishing steps add significant cost. Polishing requires expensive equipment, substantial maintenance of the equipment, and significant handling. It is typically accomplished using a slurry containing cerium (in the form of cerium oxide, Ce2O3), an expensive rare earth element. Because two polishing steps are conventionally used, two polishing machines (or sets of machines) are required, and disks must be removed from one machine, thoroughly cleaned of all slurry, and loaded onto the second machine, to complete the polishing process.
Glass disks are currently significantly more expensive than conventional aluminum disks. Unless the cost of glass disk manufacture can be substantially reduced, it will be difficult to replace aluminum with glass and realize the potential benefits that glass disks offer.
In accordance with the present invention, the flat, data recording surfaces of glass or ceramic disk substrates for use in disk drive data storage devices are polished in a process which uses a single load of the disks to a polishing apparatus and a single polishing slurry. Preferably, the process varies at least one polishing parameter at multiple stages to achieve both a reasonable rate of removal during one stage and a smooth finished surface during another stage.
In the preferred embodiment, the substrate material is glass. The polishing slurry is a cerium oxide slurry having a grit approximating that used in a conventional second (fine) polishing step. A polishing pad has surface characteristics intermediate those of a relatively hard pad typically used for the initial rough polish step, and of a relatively soft pad typically used for the second fine polish step. After loading in the polishing machine, the pressure and speed of the polishers are gradually ramped up to high levels. The polisher operates at high pressure and speed during a material removal stage. When sufficient material has been removed, the polisher reduces speed and pressure during a finishing stage to achieve a suitable surface finish. The disks are not removed from the machine between the two stages, and the machine need not be stopped.
In the preferred embodiment, the disks are lapped before being subjected to polishing. The first stage (material removal stage) continues sufficiently long to remove the entire fracture layer left by the lapping process. Alternatively, the disks are not lapped after glass forming, and the first stage (material removal stage) is used instead to remove surface waviness in the disks.
By using a polishing process in accordance with the present invention, the number of polishing machines required is reduced, an intermediate cleaning step is unnecessary between two polishes, and disk handling is reduced, all contributing to a lowered cost of manufacture.
The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: